Because the ship is not fitted with any major anti-sub systems, being that it's an anti-aircraft destroyer, it can have a merlin or lynx fitted with torpedos though which some people overlook and under estimate.

Or because folks are getting confused with Sub Harpoon, which is basically the same weapon though fired from a submarine via one of the tubes and a big white missile tube type sabot thing. Anyway, from a boats perspective a stealth or fire and forget weapon it is not.